This invention relates to an hydrophone arrangement within a towed streamer.
This invention relates to hydrophone arrangements within towed streamers which are generally solid filled rather than the more common form of array which is liquid filled.
The term hydrophone is used in this specification to include acoustic sensors which are positioned within such towed streamers.
Towed streamers often comprise several sections, each having its own function and characteristics. The active section of the streamer contains most acoustic sensors which sensors are placed in fixed relationship to each other behind the towing vessel at an appropriate depth. For the sensor to be effective in detecting acoustic waves generated in the water from sources of interest, it is desirable to ensure that the hydrophone is as free from extraneous noise generated by the environment and the towing system as is possible.
Typical noise sources compose sea noise generated by wave action and marine animals, noise produced by water flow over all elements of the towed array and vibration induced excitation from the towing vessel and cross flow over the tow cable sections which are towed at an angle to the direction of tow.
There is associated with each noise source a spectrum of waves which can be characterised by wave speed and frequencies commonly referred to as wavenumber spectrum. The wave excitation originating at sources excite waves in the towed streamer structure of the active section containing the sensors and indeed the waves can be generated within the sensor itself, such waves will be referred to as structural waves.
The region of interest for acoustic targets exists over only a small region of the wavenumber spectrum and is characterized by a single wave speed whereas the noise tends to be from a wider region. Should any of the structural waves existing in the towed streamer be within the acoustic wavenumber window of interest then any genuine acoustic target coinciding with the structural waves will be masked by the presence of such structural waves.
There are two methods of eliminating structural waves. The streamer structure can be designed in such a way that the structural waves are placed outside the acoustic window. This is normally achieved by making the structural waves slower than the acoustic wave speed. However these waves cannot, in general, be prevented from folding back into the acoustic wave space at higher frequencies.
The alternative is to make the arrangement of the hydrophones less sensitive to these structural waves. The present invention provides for a hydrophone arrangement which provides for damping or cancellation of the driving mechanisms within the hydrophone arrangement that can be excited by such structural waves. The difficulty however is to ensure that the accoustic sensitivity of the arrangement is not diminished by damping or cancelling inappropriately.
Turbulent boundary layer noise resulting from flow over the streamer, generates noise from a wide wavenumber spectrum. It is known that filtering of the turbulent boundary layer noise can be achieved by making the hydrophone acoustic window large by comparison with the flow noise wave length, that is by increasing the number of discrete sensors and appropriately spacing them. However this arrangement can also have the advantage of making the arrangement more sensitive to some structural noise but this can be counteracted through careful selection of damping.
The effectiveness of this type of filtering action can be reversed if the waves generated within the medium surrounding the acoustic sensors cannot be damped significantly at higher frequencies.
Other structural noise however also impact on the hydrophones and cannot be alleviated by such spacing arrangements.